I don't like to use shock cord (either the elastic straps or the old/new rubber bands) in the portion of the recovery anchor system that actually goes inside the body tube.
I use either Kevlar or nylon cord to attach to the motor mount or to the inside of the BT. Whatever I happen to use, I soak with a good slather of glue to penetrate the cord insides and to cover it well. Glue the cord before you build and you can reach all of it for this treatment.
I protect the cord this way because it helps make it more resistant to the momentary heat load (flux, in engineer-talk) if there is one big thick fiber with minimal surface area, as compared to many threads woven together where hot gas can get into the middle. A coating of Elmers-type glue does little to restrict any small stretch that the cord makes, but if you are really worried about it you could use another glue like silicone rubber.
Using a tether to attach to the rocket allows you to have a point outside the rocket to clip on a shock-absorbing cord, or to make replacements. It also moves the shock cord farther out the front to a zone where ejection gasses have dissipated and weakened, and do not scorch the shock cord nearly as badly.
Anyway, when the ejection gas passes through the insides of the rocket it is only there for a moment. The heat load is related to (proportional to?) ejection gas temperature, but the absorption of heat into the airframe and anchor is (I think) more related to the duration of exposure. And once the first charge goes off and the BT is empty, subsequent ejection charges go by even faster.
To protect the insides of the BT you can swab it with thinned Elmers (remember to sand lightly afterward). If you want to strengthen it significantly you can swab the insides with CA (ditto the sanding thing). Both will add significant resistance to the heat of ejection gasses.
To protect the tether cord even more, you can pull it taught so it runs straight out the front of the BT and lays against the inside wall. Add a covering/fillet of glue reaching all the way back to the anchor point but remember NOT to glue the front few inches (so you can fold the tether back and still insert the NC).
For really small rockets (BT5 and 20) where it's hard to work inside the BT, put the anchor point on the outside of the rocket. Prep for flight by inserting ejection wadding first, then the recovery device and shock cord, then pull the tether tightly up the outside of the BT and tuck it under the NC.
There are LOTS of ways to build for durability and for reliable recovery system operation, WITHOUT having to dig into motors to try to disable ejection charges.
